From Sonic Retro

Sonic Physics Guide


View Borders

The Sonic games have a screen size of 320x224 pixels during normal Zone play. The Player isn't always locked in the centre of the screen, but has some freedom of movement within the view borders before it begins to scroll.

Horizontal Border

  • Left border: 144
  • Right border: 160

If you exceed the border, the camera will move by how much the Player has exceeded, up to 16. So if they move faster than this, the camera will lag behind.

Interestingly, this centres the Player when they walk right but they aren't centred when they walk left. If you so choose, you can change the borders to 152 and 168 instead, to even things out a bit.

Sonic CD, though, effectively has both borders set to 160 so that the Player is always at dead centre and the view scrolls immediately when they begin moving.

Vertical Border

In the Air

  • Top border: 64
  • Bottom border: 128

If you exceed the border, the camera will move by how much you've exceeded, up to 16. So if the Player moves faster than this, the camera will lag behind.

Note: Knuckles uses the same vertical borders while gliding and climbing. He's considered back on the ground when he starts to clamber over the corner of a wall, or stands up after sliding from a glide (i.e. when sliding, even though he is in contact with the ground, it still considers him to be in the air).

Here's a demonstration to show how the Player's y position roams freely between the y borders.


Note: Sonic's position may appear a little lower than a border while jumping, this is due to a 5px offset while sonic is curled which applies to all characters. See Solid Tiles for more details.

On the Ground

If the Player's Y is not equal to 96, the camera will move by the difference, up to a speed of 6 (if the Player's Y speed is less than or equal to 6), or 16 (if the Player's ground speed is greater than or equal to 8). This makes the camera catch up faster when going slow, but otherwise go at a slow pace.

Here's a demonstration to show how the Player's y position stays locked to the central y.


Looking Up and Down

  • Looking Up: Scrolls up by 104 pixels, at a rate of 2 per step.
  • Looking Down: Scrolls down by 88 pixels, at a rate of 2 per step.

In Sonic 1, the camera begins to scroll immediately when you press up or down. Obviously, this is annoying if there is to be a spindash or super peel out in your engine. In Sonic 2, 3 & K, the solution is to simply wait 120 steps after you begin to hold the button before it starts scrolling. In Sonic CD, though, another method is employed. After you press the up or down button, you have 16 steps to press it again, upon which the screen begins to scroll. Unfortunately it will freeze the scrolling if you press the up or down button while the screen is already scrolling. For instance, look up by double-tapping, let go, and then press down. The screen will freeze, and stay there until you let go. Even initiating a spindash doesn't return things to normal.

In all the games, once you let go of up or down (also in Sonic 2, 3, & K, once you initiate a spindash), the camera scrolls back to the neutral position by 2 pixels per step.

Spindash Lag

(Not in Sonic CD)

The Player object has a previous position table in memory. The game knows where they've been over the last several seconds with frame precision. This is called upon for the spindash lag.

When the spindash is launched, a timer (t) is set to 32 (minus the rev variable, which can be between 0 and 8 - see spindash for more details). t then decreases by 1 every step.

While t is non-zero, the camera stops behaving normally and begins to move, not based on the Player's current position, but their position t-1 steps ago. Once t runs out, the camera returns to business as usual.

If the camera travels 32 steps into the past, and moves based on those recorded positions for a duration of 32 steps, it would effectively just repeat them, and then switch back to the current position, hopping 32 steps back into the future. This isn't how it works. Since it goes back 32 steps, and waits 32 more to return to normal, that means a total of 64 recorded camera positions. In order to take all of these positions into account during the 32 steps before the camera returns to normal, they are added together in pairs.

As an example, let's imagine the Player has been charging up their spindash for at least 32 steps, so that they haven't moved during this time. Then, they launch at a speed of 8. Since in the last 32 steps he hasn't moved, the camera will move by 0 + 0 for 16 frames, remaining stationary. Then, it will have caught up to the point in time at which the Player launched. The Player will have been moving 8 pixels every step from this point on. The camera will then move 16 pixels for 16 more steps until t runs out. (Technically, the Player doesn't move in the exact frame in which the spindash launches, so the camera will move by 0 + 8 for one step, and then 8 + 8 for a while, and the 7 + 8 as friction kicks in, and then 7 + 7, and so on).

The trouble with this lag is that if you initiate and release a spindash quickly enough after moving, the camera will actually move backward to follow where you've been.

Flame Shield/Hyper Sonic Air Dash

The same lag routine happens when Sonic does the airdash as Hyper Sonic, or with the Flame Shield. However, the trick is when he launches, the entire previous position table is blanked out with his current position, so that the camera can't scroll backward. In your engine, if you do the same thing for the launch of a spindash, you won't have to worry about backward scrolling at all.


  • You can achieve an almost identical effect without a previous position table by simply disallowing the camera to move for about 16 steps after launching. This is somewhat easier to do.

Extended Camera (Sonic CD)

In Sonic CD only, when the Player reaches a ground speed of 6 (or is 16 steps into charging up a spindash or super peel out), the camera's X position begins to shift forward. It moves 64 pixels in total, 2 per step. When their ground speed drops back below 6, it moves back. The issue is, the ground speed doesn't update in mid-air, often leading to an off-centre view when jumping or running off of a ledge.

If you use this shift effect in your engine, you could perhaps use the Player's actual horizontal speed instead, as it doesn't make sense to scroll horizontally when going down or up a wall, or worse, on a ceiling.